Composite sound-absorbing device with built in resonant cavity

ABSTRACT

The composite sound-absorbing device of the present invention includes a perforated board having a number of first pores thereon, a back board and side boards, the perforated board, back board and side boards forming a closed cavity, wherein: at least one or more of the resonant cavities being located within the closed cavity; at least one or more of second pores being located on the resonant cavities; at least one of the second pores being connected with the closed cavity; the resonant cavity having a volume of V=10 mm 3  −1×10 10  mm 3 , having a thickness of 0.05 mm-10 mm, the second pores having an aperture of d′=0.05-100 mm, with a perforation rate σ′=0.01%-30%. The present invention is beneficial to improve the effect of sound-absorbing and expand the frequency band of sound-absorbing.

FIELD OF THE INVENTION

The present invention relates to a composite sound-absorbing device andmore particularly relates to a composite sound-absorbing device withbuilt-in resonant cavity.

BACKGROUND OF THE INVENTION

In noise control engineering, many types of sound-absorbing material andstructures are used, which can be roughly divided into poroussound-absorbing materials and resonant sound-absorbing materialsaccording to their acoustical principles. For example, fiber materialsand plaster materials, among others, fall into the category of poroussound-absorbing materials, while resonant sound-absorbing structure ofthin board, resonant sound-absorbing structure of membrane and resonantsound-absorbing structure of perforated board fall into the category ofresonant sound-absorbing materials. In 1975, Dah-You Maa published anarticle titled “Theory and Design of Microperforated boardSound-absorbing Structure” published in Science in China and in 2000“Theory of Micro slit Absorbers” in Chinese Journal of Acoustics,wherein Maa expanded the application range of resonant sound-absorbingstructure.

Although resonant sound-absorbing structure of perforated board,resonant sound-absorbing structure of microperforated board and doublelayer microperforated sound-absorbing structure are superior to poroussound-absorbing material in terms of sound absorption characteristics,flow resistance, anti-moisture, anti-corrosion and hygiene, they stillcannot meet some practical needs of noise control engineering,especially when dealing with low frequency noise within strictly limitedspace for sound absorption. For as to common resonant sound-absorbingstructure, the depth of cavity has to be increased greatly to absorbmore low frequency sound, which is almost impossible to realize inpractice. Applicant has searched G10K with a special emphasis on G10k11/172 and found out “The Bundle Type Perforated board ResonantSound-absorbing Device” with patent number of CN ZL00100641.X and“Muffler with Multi Insert Pipe Parallel Connected Structure” withpatent number of CN ZL00264613.7.

The bundle type perforated board resonant sound-absorbing devicefeatures a bundle type perforated board resonant sound-absorbingstructure, which is consisted of a perforated board, a bottom board andside board (forming a closed cavity) and a bundle of tubes. The diameterof the tubes is equal to that of the pores on the perforated board andthe length of these tubs is not restrained by the cavity depth of theperforated board resonant sound-absorbing device. The tubes can eitherbe longer or shorter than the cavity depth so as to tune resonancefrequency and alter sound absorption coefficient. This sound-absorbingstructure is designed on the basis of the sound-absorbing principle ofcoupling resonance to increase its sound absorption coefficient,acoustic impedance and to enhance the sound-absorbing effect of lowfrequency sound. However this structure absorbs only sound within lowand medium frequency band, which band is not wide enough. The length ofthose flexible tubes is critical in that if the tubes are not longenough, the sound-absorbing performance would be greatly affected, i.e.,greatly degrading sound-absorbing effect. Therefore longer tubes have tobe used to ensure good sound-absorbing performance. Accordingly cavityhas to be deeper correspondingly. However longer tubes and deepercavities are not beneficial to expand the application range of thisstructure. It is further compounded by the fact that the tubes beingwire like, this structure cannot give full play the coupling resonanceeffect of tube cavity. Moreover, the length of the tubes contributesless to the consumption of acoustic energy.

The muffler with multi insert pipe parallel connected structuredescribed in ZL00264613.7 is designed for the intake system for internalcombustion engine of automobiles and that it includes an intake pipe andtwo or four resonant cavities arranged in parallel. The resonantcavities are arranged in a casing. Each of the resonant cavities isconnected to a radial-direction pore axially arranged on the intakepipe, through conduct pipes. The size of the radial-direction pore andthe conduct pipes is designed to match with the intake noise spectrum ofthe internal combustion engine. This muffler is not only able to greatlyreduce the intake noise but also increase the power of the internalcombustion engine. Moreover, it is compact in size.

Therefore, it has been a long-time effort internationally in the fieldof acoustics and noise control engineering to invent a device, which caneffectively absorb low frequency sound and has a wide sound-absorbingfrequency band to replace or improve conventional sound-absorbingstructure which is deficient in absorbing low frequency sound. To thisend, this invention proposes a composite sound-absorbing device withbuilt-in resonant cavity. This device is realized based on severalprinciples, namely by combining acoustic scattering inside the resonantcavity, sound elimination of small pores and the coupling resonance ofmultiple resonant cavities, to increase sound absorption coefficient andexpand sound frequency band.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the defect of theabove sound-absorbing structure used in current noise controlengineering that it cannot absorb enough sound with low and mediumfrequency by providing a composite sound-absorbing structure withbuilt-in resonant cavity.

According to the present invention, a composite sound-absorbing devicewith a built-in resonant cavity, includes: a perforated board having anumber of first pores thereon, a back board and side boards, theperforated board, back board and side boards forming a closed cavity,wherein: at least one or more of the resonant cavities being locatedwithin the closed cavity; at least one or more of second pores beinglocated on the resonant cavities; and at least one of the second poresbeing connected with the closed cavity; the resonant cavity having avolume of V=10 mm³-1×10¹⁰ mm³, the thickness of the wall thereof being0.05 mm-10 mm, the second pores having an aperture of d′=0.05-100 mm,with a perforation rate σ′=0.01%-30%.

In the composite sound-absorbing device of the present invention, theresonant cavity is in a shape of sphere, ellipsoid or polyhedron.

Furthermore, in the composite sound-absorbing device of the presentinvention, the second pores are connected to the closed cavity directly,or are connected to the closed cavity via tubes. Moreover, in thecomposite sound-absorbing device of the present invention, when thenumber of the resonant cavities is more than one, they are locatedwithin the closed cavity directly or fixed separately within the closedcavity partitioned by a number of partition boards.

Preferably, in the composite sound-absorbing device of the presentinvention, the first or second pores are connected to one end of thetubes and the tubes are located within the closed cavity for increasingacoustical impedance. Preferably, in the composite sound-absorbingdevice of the invention, the other end of the tubes on the second poresare connected to the closed cavity, the second pores on another resonantcavity or the first pores on the perforated board.

Preferably, in the composite sound-absorbing device of the presentinvention, the tubes are made of metal, glass, plastic or rubber; whenthe tubes are made of rubber, they are connected to the first pores orsecond pores via binding, or they are connected to the first pores via afirst transition joint at the ends of the tubes, or they are connectedto the second pores via a second transition joint at the ends of thetubes; when the tubes are made of metal, glass or plastic, they areconnected to the first pores or second pores via binding, welding,thread connection or injection, or they are connected to the first poresvia a first transition joint at the ends of the tubes, or they areconnected to the second pores via a second transition joint at the endsof the tubes.

Preferably, in the composite sound absorptive device of the presentinvention, the perforated board has a thickness of 0.5-10 mm, thediameter of the first pores on the perforated board d is 0.1-5 mm, witha perforation rate of 0.1%-30%. Furthermore, the first pores on theperforated board are arranged regularly in a shape of triangle or squareor irregularly. Moreover; the closed cavity has a depth D of 10-2000 mm,and is in a shape of cylinder composed by one side board or a polyhedroncomposed by a plurality of side boards.

Preferably, in the composite sound-absorbing device of the presentinvention, the back side of said perforated board is coated with a layerof porous sound-absorbing material, the layer of porous sound-absorbingmaterial being located within the closed cavity, with a thickness of 0.1mm-200 mm.

In the above technical solutions, the perforated board can be ironboard, steel board, copper board, corrosion resistant board, aluminumboard, plastic board, glass board, PVC board, PE board or wood board.

In the above technical solutions, the resonant cavity can be made ofmetal, glass, ceramics, rubber, plastic or fiber. The length L of thetubes is 1-5000 mm. The diameter of the tubes is 0.1-100 mm.

The present composite sound-absorbing device with built-in resonantcavity comprises a perforated board with a plurality of pores, a backboard, side board(s) and multiple resonant cavities. The resonantcavities are small cavities placed in a closed cavity. The resonantcavities are used to scatter sound, connect with the closed cavity andincrease acoustic impedance. When a sound wave reaches the resonantcavities, the air inside the cavity vibrates back and forth. Due toviscous damping, part of the acoustic energy is converted into heatenergy and is lost. By using the principle of Helmholtz resonator, thepores on the wall of the resonant cavities increase acoustic impedanceof the perforated board, sufficiently consume the acoustic energy and soenhance sound absorption. The fact that the resonant cavity being hollowincreases acoustic resistance of the present sound-absorbing device. Atthe same time, the resonant cavities are connected with the closedcavity serially so as to realize multiple cavities' coupled resonance,thereby expanding the frequency band of sound absorption consequently.Furthermore, the size of each of the resonant cavities can be differentfrom each other and the size of each of the second pores can bedifferent from each other in order to tune the resonant frequency andalter sound absorption coefficient under different frequencies. Thepresent invention utilizes the resonant cavity to scatter sound in theclosed cavity and utilizes the second pores to increase acousticimpedance and consume acoustic energy. In addition, the presentinvention modulates formant and sound-absorbing frequency band based onthe principle of multiple-cavity coupled resonance. Therefore, thepresent invention increases acoustic impedance, improves sound qualityand the effect of sound absorption and expands sound-absorbing frequencyband.

Major technical features of the present invention include: the resonantcavity is connected with the closed cavity via second pores to realizecoupling resonance among cavities and so expand sound-absorbingfrequency band. In addition, there is no limitation imposed on thenumber of the pores on the resonant cavity, thus increasing acousticimpedance of the sound-absorbing device. The number of the pores and thediameter of the pores can be adjusted as required to increase or reducethe acoustic impedance and thus to increase sound absorptioncoefficient. The tubes connecting to the resonant cavities increase thethickness of the pores on the resonant cavities, which is not only tothe benefit of increasing acoustic impedance but also realizes couplingresonance by connecting the tubes with resonant cavities. Moreover, thepresent invention advantageously can increase sound absorptioncoefficient, expand sound-absorbing frequency band and cause the soundabsorption frequency band to shift towards low frequency band, so it isbeneficial to absorb sound with low frequency. With the coupledresonance of the resonant cavities and the closed cavity, it can beregarded that sound absorption is carried out in a double-deck structurewithin the same and one cavity. In the meantime, the capacity of therear cavity is reduced. Therefore, the present invention is suitable tothe situations where space for sound absorption is strictly limited.Moreover, in order to expand the frequency range of noise elimination ofthe present composite sound-absorbing device, each of the resonantcavities can be different from each other in size and shape, and each ofthe second pores can be different from each other in size and shape,which is beneficial for the present invention to be used in differentsound elimination situations. The acoustic scattering on the surfaces ofthe resonant cavities allows the sound wave to reach to every resonantcavity in the rear cavity and pushes the air in the second pores tovibrate back and forth, thereby consuming acoustic energy sufficientlyand being beneficial to absorb sound by using the space of the rearcavity.

The advantages of the invention lie in that, by arranging a plurality ofresonant cavities in the limited space of the rear cavity, the presentinvention makes full use of the principles of acoustic scattering,pores' acoustic impedance consuming acoustic energy and sound absorptionby multi-cavity coupled resonance, as well as the modulation features ofthe size of the cavities and the pores to formant and sound-absorbingfrequency band, thus increasing sound absorption coefficient, enhancingthe absorption of low and medium frequency noise and expandingsound-absorbing frequency band.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a composite sound-absorbing device withbuilt-in resonant cavities of the present invention, wherein each of theresonant cavities has a second pore connecting directly with a closedcavity;

FIG. 2 schematically shows another embodiment of the compositesound-absorbing device of the present invention, wherein each resonantcavity has 26 second pores connecting with a closed cavity;

FIG. 3 schematically shows another embodiment of the compositesound-absorbing device of the present invention, wherein each resonantcavity has four second pores, and one of the second pores connects withone first pore on a perforated board via a tube, while the other secondpores connect with a closed cavity directly;

FIG. 4 is still another embodiment of the composite sound-absorbingdevice according to the present invention, wherein each resonant cavityhas three second pores, one of which connects with a closed cavity viatubes;

FIG. 5 is still another embodiment of the composite sound-absorbingdevice according to the present invention, wherein each resonant cavityhas two second pores, and for every two resonant cavities there areconnected tubes therebetween, the other second pores directly connectswith a closed cavity;

FIG. 6 schematically shows an embodiment of the compositesound-absorbing device according to the present invention, wherein afirst transit joint and a second transit joint are installed;

FIG. 7 is another embodiment of the composite sound-absorbing deviceaccording to the present invention, wherein each of the resonantcavities has two second pores with different diameters;

FIG. 8 is another embodiment of the composite sound-absorbing deviceaccording to the present invention, wherein two resonant cavities withdifferent volumes are arranged in a closed cavity;

FIG. 9 is another embodiment of the composite sound-absorbing deviceaccording to the present invention, wherein ellipsoid resonant cavitiesand cubic resonant cavities are arranged in a closed cavity;

FIG. 10 schematically shows an embodiment of the compositesound-absorbing device according to the present invention, whereinpartition boards are installed;

FIG. 11 is still another embodiment of the composite sound-absorbingdevice of the present invention, wherein first pores on a perforatedboard connect with tubes;

FIG. 12 is another embodiment of the composite sound-absorbing device ofthe present invention, wherein the back side of the perforated board iscovered with a layer of porous sound-absorbing material;

FIG. 13 is a comparison chart showing the sound-absorbing performance ofresonant sound-absorbing device of the present invention and aperforated board (Cavity depth is 50 mm), by using a standing wavemeter;

FIG. 14 is a comparison chart showing the sound-absorbing performance ofdifferent composite sound-absorbing devices with different number ofresonant cavities(cavity depth is 100 mm) according to the presentinvention, by using a standing wave meter; and

FIG. 15 is a comparison chart showing low and medium frequency soundperformance of a composite sound-absorbing device with built-in resonantcavity, a perforated board with tubes and a perforated board(cavitydepth is 50 mm), by using a standing wave meter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, the present invention will be described in detailswith reference to the accompanying drawings and embodiments.

Embodiment One

Referring to FIG. 1, the embodiment provides a composite sound-absorbingdevice with built-in resonant cavity. The device comprises a closedcavity formed by a perforated board 1, a back board 2 and side boards 3all made up of stainless steel, wherein the depth D of the closed cavityis 40 mm. The perforated board 1 is a square board with the length ofthe side being 80 mm and the thickness being 5 mm. On the perforatedboard 1, first pores 6, with a diameter of 3 mm, are formed. Theperforation rate σ of the first pores 6 is 28%. The first pores 6 areregularly arranged in the pattern of a square on the perforated board 1.In the closed cavity, four resonant cavities 5 are formed, with eachresonant cavity 5 being made of aluminum and having a shape of sphere.The volume of the resonant cavity 5 is 1.4×10⁴ mm³ and the thickness ofthe wall of the resonant cavity 5 is 5 mm. Moreover, on the wall of theresonant cavity 5, a second pore 6′, with a diameter of 2 mm, is formed.The perforation rate σ′ of the second pore 6′ is 0.06%. The resonantcavity 5 is arranged in the closed cavity freely.

Embodiment Two

Referring to FIG. 2, the present embodiment provides a compositesound-absorbing device with built-in resonant cavity according to thepresent invention. The device comprises a closed cavity formed by aperforated board 1, a back board 2 and side boards 3 all made ofstainless steel, wherein the depth D of the closed cavity is 50 mm. Theperforated board 1 is a round board, with a diameter of 100 mm and athickness of 0.7 mm. On the perforated board 1, first pores 6, with adiameter of 1.7 mm, are formed. The perforation rate σ of the firstpores 6 is 4.6%. The first pores 6 are arranged regularly in the patternof a square on the perforated board 1. In the closed cavity, fourresonant cavities are formed, with each resonant cavity being made ofplastic. The volume of the resonant cavity 5 is 3.35×10⁴ mm³ and thethickness of the wall of the resonant cavity 5 is 0.4 mm. Furthermore,there are 26 second pores 6′ on the wall of the resonant cavity 5,evenly distributed on the circumferences of three mutually perpendicularhemispheres. (There are 16 second pores 6′ on each hemisphericalcircumference, with 4 second pores 6′ overlapping on every twohemispherical circumferences), the diameter d′ of the second pores 6′being 0.5 mm and the perforation rate σ′ being 0.1%. The resonantcavities 5 are arranged in the closed cavity freely.

An experiment was conducted to test low and medium frequency soundmuffling mechanism of the composite sound-absorbing device with built-inresonant cavity by using a standing wave meter. In the experiment, thelow and medium frequency sound absorption coefficient of a perforatedboard, a perforated board whose cavity is provided with sphere withoutpores and a composite sound-absorbing device with built-in resonantcavity are measured to verify that multiple cavities coupling isbeneficial to increase sound absorption coefficient. Other parameters ofresonant sound-absorbing structures employed in the experiment arelisted as follows:

Parameters of the perforated board: the pores are arranged in thepattern of a square, with the diameter of the pores being 1.7 mm, thecenter to center spacing of the pores being 7 mm, the thickness of theperforated board being 0.7 mm and the depth of the closed cavity being50 mm.

Parameters of the perforated board whose cavity is provided with spherewithout pores: the pores are arranged in the pattern of a square, withthe diameter of the pores being 1.7 mm, the center to center spacing ofthe pores being 7 mm, the thickness of the perforated board being 0.7mm. Four plastic hollow spheres without pores are placed in the closedcavity, with the thickness of the wall of the sphere being 0.4 mm andthe volume V of the sphere being 3.35×10⁴ mm³. The spheres are arrangedin the closed cavity freely, with the depth of the closed cavity being50 mm.

FIG. 13 shows that the sound absorption coefficient of the perforatedboard and the perforated board with built-in spheres without pores issimilar to each other, with the highest sound absorption coefficientbeing no greater than 0.35 at the frequency band of 1000 Hz and 1250 Hz,i.e., the sound-absorbing effect of these two devices is not desirable.As to the composite sound-absorbing device with built-in resonantcavity, its formant reaches 0.928 at the frequency of 630 Hz and reachesabove 0.5 at the frequency band of 500 Hz and 1250 Hz(i.e., the bandwidth is 750 Hz). From the above, it is apparent that thesound-absorbing effect of the composite sound-absorbing device withbuilt-in resonant cavity is superior to the other two.

Embodiment Three

Referring to FIG. 2, the embodiment provides a composite sound-absorbingdevice with built-in resonant cavity. The device comprises a closedcavity formed by a perforated board 1, a back board 2 and side boards 3all made up of stainless steel, with the depth D of the closed cavitybeing 100 mm. The perforated board 1 is a round board, with a diameterof 100 mm and thickness of 0.7 mm. On the perforated board 1, firstpores 6, with a diameter of 1.7 mm are formed. The perforation rate ofthe first pores 6 is 4.6%. The first pores 6 are arranged regularly in apattern of square on the perforated board 1. Separately, nine, seven,four and one resonant cavity 5, made of plastic and having a shape ofsphere and a volume V of 3.35×10⁴ mm³ and the thickness of the wall ofthe resonant cavity 5 being 0.4 mm, is arranged in the closed cavity.Furthermore, there are 26 second pores 6′ on the wall of the resonantcavity 5, evenly distributed on the circumferences of three mutuallyperpendicular hemispheres (There are 16 second pores 6′ on eachhemispherical circumference, with 4 second pores 6′ overlapping forevery two hemispherical circumferences). The second pores 6′ have adiameter d′ of 0.5 mm and the perforation rate σ′ of the second pores 6′is 0.1%. The resonant cavities 5 are arranged in the closed cavityfreely.

In the experiment, four composite sound-absorbing devices with built-inresonant cavity according to the present invention are separatelyprovided with nine, seven, four and one resonant cavity inside theclosed cavity. The experiment tests the low and medium frequency soundmuffling mechanism by using a standing wave meter to verify the impactof the number of resonant cavities on sound absorption coefficient andthe frequency band of sound absorption. The other parameters of theresonant sound-absorbing structures employed in the experiment arelisted as follows:

Parameters of the perforated board: the pores, with a diameter of 1.7mm, are arranged in a pattern of square, with the center to centerspacing of the pores being 7 mm, the thickness of the board being 0.7 mmand the depth of the closed cavity being 100 mm. From FIG. 14, it isknown that, the sound absorption coefficient of the resonantsound-absorbing device with one resonant cavity is no greater than 0.4at the formant of 630 Hz, and reaches about 0.6 at the frequency of 2000Hz; the sound absorption coefficient of the resonant sound-absorbingdevice with four resonant cavities is above 0.8 at the formant of 630Hz, and is greater than 0.5 at the frequency band of 500 Hz and 800 Hz,and is 0.8 at the frequency of 2000 Hz; the sound absorption coefficientof the resonant sound-absorbing device with seven resonant cavities isabove 0.95 at the formant of 800 Hz, and is greater than 0.5 at thefrequency band of 400 Hz and 800 Hz, and is about 0.85 at the frequencyof 2000 Hz; the sound absorption coefficient of the resonantsound-absorbing device with nine resonant cavities is above 0.9 at theformants of 500 Hz and 800 Hz respectively, and is greater than 0.5 atthe frequency band of 400 Hz and 1000 Hz, and is about 0.8 at thefrequency of 2000 Hz. As can be seen, as the number of the resonantcavity provided in the closed cavity increases, the frequency band isexpanded and the formant of the major sound-absorbing frequency bandbecomes bigger gradually and the number thereof increases from one totwo, whose features are similar to the sound-absorbing structure ofdouble-layer microperforated board; in addition, the sound absorptioncoefficient at the frequency of 2000 Hz increases as the number ofresonant cavities grows.

Embodiment Four

Referring to FIG. 3, the embodiment provides a composite sound-absorbingdevice with built-in resonant cavity. The device comprises a closedcavity formed by a perforated board 1, a back board 2 and side boards 3all made up of stainless steel, with the depth D of the closed cavitybeing 200 mm, 500 mm, 1000 mm or 2000 mm. The perforated board 1 is asquare board, with the length of the side being 1000 mm and thethickness thereof being 2 mm. On the perforated board 1, first pores 6,with a diameter of 2 mm, are formed. The perforation rate of the firstpores 6 is 0.031%. The first pores 6 are arranged regularly in a patternof square on the perforated board 1. In the closed cavity, 100 resonantcavities 5, made of glass and in a shape of sphere and having a volumeof 2.7×10⁵ mm³ and having a wall thickness of 10 mm, are arranged. Foursecond pores 6′, with a diameter d′ of 2 mm, are provided on the wall ofthe resonant cavity 5, evenly distributed on the circumference of ahemisphere. The perforation rate 6′ of the second pores 6′ is 0.06%.Three of the four second pores 6′ on each of the resonant cavities 5 areconnected with the closed cavity. The other second pore 6′ is connectedwith a tube 4, whose other end is connected with a first pore 6 on theperforated board 1. The tube 4 may be made of metal, rubber or glass,with a length l of 10 mm, 50 mm or 100 mm and a diameter of 2 mm. Thetubes 4 may be connected to the perforated board 1 by splicing, threadedconnection or injection mold.

Embodiment Five

Referring to FIG. 4, a composite sound-absorbing device with a built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1, a back board 2 and side board 3. The perforatedboard 1 may be made of glass, PVC, PE or wood. The back board 2 and theside boards 3 are made of glass, with the depth D of the closed cavitybeing 100 mm. The perforated board 1 is a square board with a sidelength of 200 mm and a thickness of 3 mm. On the perforated board 1,first pores 6, with a diameter of 1 mm, are provided. The perforationrate of the first pores 6 is 0.6% and the first pores 6 are arranged ina pattern of hexagon on the perforated board 1. In the closed cavity, 16resonant cavities 5, which are rubber sphere-shaped cavity, arearranged, with the volume of the resonant cavities 5 being 3.35×10⁴mm³and the thickness of the wall of the resonant cavities 5 being 0.8mm. On the wall of the resonant cavities 5, three second pores 6′ areprovided, evenly distributed on the circumference of a hemisphere. Thediameter d′ of the second pores 6′ is 1 mm and the perforation rate σ′of the second pores 6′ is 0.047%. Furthermore, the second pores 6′ ofeach resonant cavity 5 are connected with tubes 4 whose other ends areconnected with the closed cavity. The tubes 4 are made of rubber andhave a length l of 60 mm and a diameter of 1 mm. The resonant cavities 5are connected with the tubes 4 by splicing or injection molding. Theresonant cavities 5 are arranged in the closed cavity freely.

Embodiment Six

Referring to FIG. 5, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1 made of copper, a back board 2 made of stainlesssteel and side boards 3 made of stainless steel, with the depth D of theclosed cavity being 40 mm. The perforated board 1 is a square board witha side length of 80 mm and a thickness of 1 mm. On the perforated board1, first pores 6, with a diameter of 3 mm, are provided. The perforationrate a of the first pores 6 is 28%. The first pores 6 are arrangedregularly in a pattern of square on the perforated board 1. In theclosed cavity, four resonant cavities 5 made of copper and having ashape of sphere are provided, whose volume is 1.4×10⁴ mm³ and whose wallhas a thickness of 5 mm. On the wall of the resonant cavities 5, twosecond pores 6′, with a diameter d′ of 5 mm, are provided. The secondpores 6′ are evenly distributed on the circumference of a hemisphere.The perforation rate of the second pores 6′ a′ is 1.4%. Every tworesonant cavities 5 form a group and are connected with two second pores6′ on two resonant cavities through tubes 4 and the other second pores6′ are connected with the closed cavity, as shown in FIG. 5. The tubes 4are made of steel and have a length of 5 mm and a diameter of 5 mm. Thetubes 4 and the perforated board 1 are connected by splicing, threadedconnection or injection molding and the resonant cavities are connectedwith the tube 4 by welding or threaded connection. The resonant cavities5 are arranged in the closed cavity freely.

Embodiment Seven

Referring to FIG. 3 and FIG. 6, a composite sound-absorbing device withbuilt-in resonant cavity is provided. The device comprises a closedcavity formed by a perforated board 1 made of plastic, a back board 2made of stainless steel and side boards 3 made of stainless steel, witha depth D of 200 mm. The perforated board 1 is a square board with aside length of 1000 mm and has a thickness of 2 mm. On the perforatedboard 1, first pores 6, with a diameter of 2 mm, are provided. Theperforation rate of the first pores is 0.031%. The first pores 6 arearranged regularly in a pattern of square on the perforated board 1. Inthe closed cavity, one hundred resonant cavities 5, which is in a shapeof sphere and made of plastic and having a volume V of 2.7×10⁵ mm³, arearranged. The thickness of the wall of the resonant cavities 5 is 10 mm.On the wall of each of the resonant cavities 5, two second pores 6′,with a diameter d′ of 2 mm, are provided. The second pores 6′ are evenlydistributed on the circumference of a hemisphere. The perforation rateσ′ of the second pores 6′ is 0.03%. One second pore 6′ of each resonantcavity 5 is connected with the closed cavity and the other second pore6′ is connected with a tube 4 whose other end is connected with a firstpore 6 on the perforated board 1. The tubes 4 are made of rubber andhave a length of 100 mm and a diameter of 2 mm. The perforated board 1is connected with the tubes 4 by using a first transit joint 7 and theresonant cavities 5 are connected with the tubes by using a secondtransit joint 7′.

Embodiment Eight

Referring to FIG. 7, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1 made of plastic, a back board 2 made ofstainless steel and side boards 3 made of stainless steel, with a depthD of 200 mm. The perforated board 1 is a square board with a side lengthof 1000 mm and a thickness of 2 mm. On the perforated board 1, firstpores 6, with a diameter of 2 mm, are provided. The perforation rate ofthe first pores 6 is 0.031%. The first pores 6 are arranged regularly ina pattern of square on the perforated board 1. In the closed cavity, onehundred resonant cavities 5, which are in a shape of sphere and made ofplastic and have a volume V of 2.7×10⁵ mm³, are arranged. The thicknessof the wall of the resonant cavities 5 is 2 mm. On the wall of each ofthe resonant cavities 5, two second pores 6′, one of which has adiameter d′ of 3 mm and the other has a diameter d′ of 1 mm, are notevenly distributed on the circumference of a hemisphere. The perforationrate σ′ of the second pores 6′ is 0.039%. The resonant cavities 5 arearranged in the closed cavity freely.

Embodiment Nine

Referring to FIG. 8, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1 made of copper, a back board 2 made of stainlesssteel and side boards 3 made of stainless steel, with the depth D of theclosed cavity being 40 mm. The perforated board 1 is a square board witha side length of 80 mm and a thickness of 1 mm. On the perforated board1, first pores 6, with a diameter of 3 mm, are provided. The perforationrate of the first pores 6 is 28%. The first pores 6 are arrangedregularly in a pattern of square on the perforated board 1. In theclosed cavity, four resonant cavities 5, which are in a shape of sphereand made of plastic, are arranged. On the wall of each of the resonantcavities 5, three second pores 6′ are provided, which are evenlydistributed on the circumference of a hemisphere. The thickness of thewall of the resonant cavities 5 is 1 mm. Two resonant cavities 5 have avolume of 3.3×10⁴ mm³ and the diameter of the second pores 6′ thereon is2 mm and the perforation rate of the second pores 6′ thereon is 0.19%,and the other two resonant cavities 5 have a volume of 8.3×10³ mm³ andthe diameter of the second pores 6′ thereon is 1 mm and the perforationrate of the second pores 6′ thereon is 0.12%. The resonant cavities 5are arranged in the closed cavity freely.

Embodiment Ten

Referring to FIG. 8, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity,formed by a perforated board 1 made of copper, a back board 2 made ofstainless steel and side boards 3 made of stainless steel, with thedepth D of the closed cavity being 40 mm. The perforated board 1 is asquare board with a side length of 80 mm and a thickness of 1 mm. On theperforated board 1, first pores 6, with a diameter of 3 mm, areprovided. The perforation rate of the first pores 6 is 28%. The firstpores 6 are arranged regularly in a pattern of square on the perforatedboard 1. In the closed cavity, four resonant cavities 5 made of plasticare arranged, wherein the thickness of the wall thereof is 0.5 mm. Onthe wall of each of the resonant cavities 5, one second pore 6′ isprovided. Among the four resonant cavities 5, two are ellipsoid having avolume of 3.3×10⁴ mm³ and the diameter of the second pores 6′ on them is2 mm and the perforation rate of the second pores 6′ is 0.063%, theother two are cubic having a volume of 6.4×10⁴ mm³ and the diameter ofthe second pores 6′ on them is 2 mm and the perforation rate of thesecond pores 6′ is 0.03%. The resonant cavities 5 are arranged in theclosed cavity freely.

Embodiment Eleven

Referring to FIG. 10, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1, a back board 2 and side boards 3 all made up ofstainless steel, with the depth D of the closed cavity being 40 mm. Theperforated board 1 is a square board with a side length of 80 mm and athickness of 5 mm. On the perforated board 1, first pores 6, with adiameter of 3 mm, are provided. The perforation rate of the first pores6 is 28%. The first pores 6 are arranged regularly in a pattern ofsquare on the perforated board 1. In the closed cavity, four resonantcavities 5 made of plastic are provided, wherein the resonant cavities 5are in shape of a sphere with a volume of 942 mm³ and the thickness ofthe wall of the resonant cavities 5 is 1 mm. On the wall of each of theresonant cavities 5, one second pore 6′, with a diameter of 2 mm, isprovided. The perforation rate σ′ of the second pores 6′ is 0.7%.Furthermore, partition boards are installed inside the closed cavity,thereby separately fixing the four resonant cavities 5.

Embodiment Twelve

Referring to FIG. 11, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1, a back board 2 and side boards 3 all made up ofstainless steel, with the depth D of the closed cavity being 50 mm. Theperforated board 1 is a round board with a diameter of 100 mm and athickness of 0.7 mm. On the perforated board 1, first pores 6, with adiameter of 1.1 mm, are provided. The perforation rate of the firstpores 6 is 1.9%. The first pores 6 are arranged regularly in a patternof square on the perforated board 1. In the closed cavity, four resonantcavities 5 made of plastic are provided, wherein the resonant cavities 5are in shape of a sphere having a volume of 3.35×10⁴ mm³ and thethickness of the wall thereof is 0.4 mm. Moreover, there are twenty-sixsecond pores 6′, with a diameter of 0.5 mm, on the wall of the resonantcavities 5, evenly distributed on the circumferences of three mutuallyperpendicular hemispheres (There are 16 second pores on eachhemispherical circumference, with 4 second pores overlapping for everytwo circumferences). The perforation rate σ′ of the second pores 6′ is0.1%. The resonant cavities 5 are arranged freely in the closed cavity.Each of the first pores 6 on the perforated board 1 is connected with astainless steel tube 4, which is 8.5 mm long and has a diameter of 1.1mm. The tubes 4 are welded on the first pores 6 of the perforated board1.

A comparison experiment is conducted to verify the sound mufflingmechanism of low and medium frequency sound of the compositesound-absorbing device according to the present invention and theperforated board with tubes by using a standing wave meter. In theexperiment, the low and medium sound absorption coefficient of theperforated board, the perforated board with tubes and the compositesound-absorbing device with built-in cavities are measured respectivelyto determine the effect of resonant cavities provided in the perforatedboard sound-absorbing structure. The other parameters of the resonantsound-absorbing structure are listed as follows:

Parameters of the perforated board: the pores are arranged in a patternof square, with the diameter of the pores being 1.7 mm, the center tocenter spacing of the pores being 7 mm, the thickness of the wall of theperforated board being 0.7 mm and the depth of the cavity being 50 mm.

Parameters of the perforated board with tubes: the pores are arranged ina pattern of square, with the diameter of the pores being 1.1 mm, thecenter to center spacing of the pores being 7 mm, the thickness of thewall of the perforated board being 0.7 mm, the length of the tubes being8.5 mm and the diameter of the tubes being 1.1 mm. The tubes are weldedon the pores on the perforated board. The depth of the cavity is 50 mm.

As shown in FIG. 15, in comparison with the perforated board, the mainresonance frequency band of the perforated board sound-absorbingstructure with tubes and the composite sound-absorbing device accordingto the present invention tend to move towards low frequency and theiraverage sound absorption coefficient is greater. In comparison with theperforated board sound-absorbing structure with tubes, thesound-absorbing formant of the composite sound-absorbing deviceaccording to the present invention is much higher and its frequency bandis wider.

Embodiment Thirteen

Referring to FIG. 12, a composite sound-absorbing device with built-inresonant cavity is provided. The device comprises a closed cavity formedby a perforated board 1, a back board 2 and side boards 3 all made up ofstainless steel, with the depth D of the closed cavity being 300 mm. Theperforated board 1 is a round stainless steel board and the diameter ofthe board is 100 mm, with a thickness of 0.8 mm. On the perforated board1, first pores 6, with a diameter of 1.1 mm, are provided. Theperforation rate of the first pores 6 is 1.9%. The first pores 6 arearranged regularly in a pattern of square on the perforated board 1. Inthe closed cavity, four resonant cavities 5, made of plastic and beingin a shape of sphere and having a volume of 3.35×10⁴ mm³, are arranged.The thickness of the wall of the resonant cavities 5 is 0.4 mm. Sixsecond pores 6′ are arranged on the wall of the resonant cavities 5,evenly distributed on the circumferences of three mutually perpendicularhemispheres. The diameter of the second pores 6′ is 0.5 mm and theperforation rate σ′ of the second pores 6′ is 0.023%. The resonantcavities 5 are arranged in the closed cavity freely. Furthermore, theback side of the perforated board 1 is covered with a layer of poroussound-absorbing material, the thickness of the layer being 0.5 mm, 5 mm,30 mm, 100 mm or 200 mm and the porous sound-absorbing material beingglass wool, foamed aluminum, foamed plastic, slag wool or cotton fiber.

To conclude, the composite sound-absorbing device with built-in resonantcavity according to the present invention makes full use of the acousticscattering on the surface of the resonant cavity, acoustic impedance ofthe second pores on the resonant cavity and the modulation to thesound-absorbing formant and sound-absorbing frequency band by resonantcavities' coupling and etc., to absorb sound, wherein itssound-absorbing frequency band is wider, sound absorption coefficient isbigger and so the absorption effect of low and medium frequency noise isimproved, when compared with conventional perforated board resonantsound-absorbing structure. Moreover, the present device is compact,economical and practical. It is clear from the above comparisonexperiments that the sound-absorbing effect of the present device isobviously superior to the perforated board resonant sound-absorbingdevice and as the number of the resonant cavities increases, the soundfrequency band becomes wider and the formant of major sound-absorbingfrequency becomes higher and gradually evolves into two formants, whichis similar to the double layer microperforated board sound-absorbingstructure. The number of resonant cavities and the pores on the resonantcavities is crucial to the present device, and if the number of theresonant cavities is not enough, the sound-absorbing effect would begreatly reduced.

It should be noted that the present invention is not necessarily limitedto the foregoing embodiments, which can be further modified in variousways within the scope of the invention as defined in the appended claim.

1. A composite sound-absorbing device with built-in resonant cavity,including: a perforated board having a number of first pores thereon, aback board and side boards, said perforated board, back board and sideboards forming a closed cavity, wherein: at least one or moreindependent said resonant cavities being located within said closedcavity randomly; said resonant cavities being in a shape of sphere,ellipsoid or polyhedron; at least one or more of second pores beinglocated on said resonant cavities; and at least one of said second poresbeing connected with said closed cavity; said resonant cavity having avolume of V=10 mm³-1×10¹⁰ mm³, the thickness of the wall of the cavitybeing 0.05 mm-10 mm, said second pores having an aperture of d′=0.05-100mm, with a perforation rate σ′=0.01%-30%.
 2. The compositesound-absorbing device of claim 1, wherein the number of said resonantcavities is more than one, which are located within said closed cavitydirectly or fixed within said closed cavity separately partitioned by anumber of partition boards.
 3. (canceled)
 4. The compositesound-absorbing device of claim 1, wherein said second pores areconnected to said closed cavity directly.
 5. The compositesound-absorbing device of claim 1, wherein said second pores areconnected to said closed cavity via tubes.
 6. The compositesound-absorbing device of claim 1, wherein said first or second poresare connected to one end of said tubes and said tubes are located withinsaid closed cavity for increasing acoustical impedance.
 7. The compositesound-absorbing device of claim 6, wherein the other end of said tubeson said second pores are connected to said closed cavity, said secondpores on another resonant cavity or said first pores on said perforatedboard.
 8. The composite sound-absorbing device of claim 6, wherein saidtubes are made of metal, glass, plastic or rubber, with a length of1-5000 mm and diameter of 0.1-100 mm; when said tubes are made ofrubber, they are connected to said first pores or second pores viabinding, or they are connected to said first pores via a firsttransition joint at the ends of said tubes, or they are connected tosaid second pores via a second transition joint at the ends of saidtubes; when said tubes are made of metal, glass or plastic, they areconnected to said first pores or second pores via binding, welding,thread connection or injection, or they are connected to said firstpores via a first transition joint at the ends of said tubes, or theyare connected to said second pores via a second transition joint at theends of said tubes.
 9. The composite sound absorptive device of claim 1,wherein said perforated board has a thickness of 0.5-10 mm, the diameterof said first pores on said perforated board d being 0.1-5 mm, with aperforation rate of 6′=0.1%-30%; said closed cavity having a depth ofD=10-2000 mm, and said closed cavity having a shape of cylinder composedby one side board or polyhedron composed by a plurality of side boards;said first pores on said perforated board are arranged in a shape ofregular triangle or square or are arranged irregularly.
 10. Thecomposite sound-absorbing device of claim 1, wherein the back side ofsaid perforated board is coated with a layer of porous sound-absorbingmaterial, said layer of porous sound-absorbing material being locatedwithin said closed cavity, with a thickness of 0.1 mm-200 mm.